CIB-W18 Timber Structures – A review of meeting 1-43 2 MATERIAL ...

– A higher COV of the raw material results in more conservative settings
– Weak correlation between indicating property and grade determining
property results in more conservative settings
– Few observations for calculation of settings results in more conservative
settings
The method is simple and based on common statistical theory. Such a
simple method is on the market today operates with multiple settings and
the prediction limit method can be adopted also to this case. The method
gives one setting for each grade independently of grade combination. Depending
on the percentile level chosen the calculated settings result in
graded material with different safety levels.
The prediction limit studied in this paper is the lower 5% limit and the
achieved characteristic alues of the graded material are compared to the
characteristic values according to EN338. The prediction limit method
opens for a possibility to calculate another limit such as the lower 1% or
lower 0,1 % limit and compare the result to other required values. It is the
combination of confidence level of the prediction limit and the required
value that defines the safety for the end user of the graded timber.
In the comparison between the achieved characteristic values after
grading based on the prediction limit and the requirements in EN338 it is
mainly the density that fails to be fulfilled. One reason for that is obviously
the poor correlation between strength and density since the settings
were determined based on the relation between IP and MOR. Other reasons
can be an unrealistically chosen density in the data sets or an unrealistic
required density in EN338.
The authors of this paper recommend a deeper analysis of the prediction
limit method for calculation of settings for grading machines. In connection
to this also new requirements for the graded material are needed.
43-5-1 F Rouger
Quality control methods - Application to acceptance criteria for a batch
of timber
Abstract
In the ratified version EN 384, a method was stated in section 9, regarding
quality verification of a batch of timber. This paper aims at showing that
the current approach is not correct, and proposes a new approach that is in
line with existing theories. This new method is based on quality control
concepts. For stiffness control, average quality criterion has to be used,
whereas for strength control, fraction defective criterion shall be applied.
Both criteria use a risk concept:
– The risk for the producer, denoted a, is the risk that a batch is rejected
by the consumer whereas it is correct
– The risk for the consumer, denoted b, is the risk that the consumer accepts
the batch whereas it is not correct
This paper firstly describes the theory, and illustrates the efficiency of the
method on simulated and real experimental data sets. It also demonstrates
the inefficiency of standardized approach. As a conclusion, the proposal
based on this investigation, and which has been incorporated in the new
version of EN 384 is briefly described.
Conclusions
Theoretical concepts of quality control have been successfully applied on
the acceptance criteria for a batch of timber. Numerical investigations on
“real data” clearly show that the existing approach of the standard is not
correct, and that the new approach gives very good results. This justifies
the revision which has been incorporated in the new version of the standard
which is going to be ratified in a close future.
43-5-2 P Stapel, J W v. d. Kuilen, A Rais
Influence of origin and grading principles on the engineering properties
of European timber
Introduction
Due to increasing effort of grading machine manufacturers the amount of
timber which can be used for approvals according to European standard
EN 14081 increases. The available data can be used to obtain uniform settings
that cover large parts of Europe. The European standard for machine
approval EN 14081-Part 2/3 was developed when much less data was
available. Alternative methods for machine grading in Europe have recently
been proposed. This paper deals with the current system. The improved
experience in deriving machine settings of today, allows a critical analysis
of the rules and methods decided upon at the time. This can be tested today,
as a dataset of almost 5000 boards is available. The approval proce-
CIB-W18 Timber Structures – A review of meeting 1-43 2 MATERIAL PROPERTIES page 2.101